Using the discrete element method coordinates of elementary acoustic emission sources (breaking bonds) were calculated as a function of time for materials of different degrees of heterogeneity under mechanical loading. The time dependences of multifractal dimension spectra D(q) for these sources were calculated using the “sandbox” algorithm. The time behavior of spectra width w and box counting fractal dimension D0 was studied. A narrowing of the multifractal dimension spectra was discovered (fractal self-organization of acoustic emission) before the destruction of materials and its subsequent expansion after the formation of a main crack. The influence of physical properties of the polycrystalline grain boundaries on the strength and ductility of materials has been revealed